It is known in the art to measure forces by means of a sensor adapted to measure difference of stresses in two orthogonal measuring directions. For example, a magnetoelastic force sensor is known from Swedish patent number 151267. The force sensor comprises a measuring body of a magnetoelastic material, and two coils enclosing at least a part of the measuring body and arranged such that the mutual inductance of the coils is essentially zero, when no force is acting on the measuring body, and one of the coils, denoted a magnetizing coil, is connected to a source of current, and the other coil, denoted a measuring coil, is connected to a voltmeter. This sensor utilizes the fact that, when a magnetostrictive material is exposed to a mechanical stress, the permeability in the stress direction is changed. The sensor measures the difference of stresses in two orthogonal measuring directions in the measuring body, which is a measure of the forces acting on the measuring body. The sensor is able to measure compressive/tensile forces, as well as shear forces, in dependence of the direction of its mounting. It is also known to measure forces by means of two orthogonally mounted strain gauges.
In order to function properly, the force sensor must be provided with appropriate stress conditions. Appropriate stress conditions are: two axial stresses in the measuring body, an appropriate relation between the two axial stresses, and an appropriate level of the stresses in the measuring body. Preferably, the relation between the two axial stresses is minus one, which means equal magnitude, but opposite directions of the stresses. The appropriate level of stresses in the measuring body depends on the type of sensor. A magnetoelastic sensor requires lower level of the stresses than a strain gauge, in order to function properly. Accordingly, it is a desire to be able to control the magnitude and direction of the stresses in the sensor.
The optimal level of stresses for the magnetoelastic force sensor is relatively low, which makes it difficult to measure large forces with this type of sensor. To be able to measure forces, which are larger than the optimal forces for the sensor, the measuring body is designed with an increased amount of material in the direction of the applied force in order to reduce the level of stresses in the measuring body. A problem with this solution is that it puts high demand on the surrounding structure, which makes it is difficult to manufacture. The large amount of material also introduces thermal problems, which affects the measurement.